Co-pending U.S. Pat. No. 4,970,682, issued Nov. 13, 1990, derived from parent application Ser. No. 168,437, filed Mar. 15, 1988, (the original `continuation chain` parent application of which, Ser. No. 224,742, now abandoned was filed Jan. 13, 1981), by Paul B. Beckwith et al, entitled Digital Map Generator and Display System, assigned to the assignee of the present application and the disclosure of which is incorporated herein, describes a digital map generation and display system which is capable of accessing and displaying a digital terrain map image to an observer (e.g. the pilot of a fixed or rotary wing aircraft) effectively in real time with the travel of the aircraft over that terrain. A principal factor in the ability of the system to effect a real time display is the combination of a scene memory, which stores a portion of digital map imagery data that has been decompressed from a mass storage device in a prescribed orientation (North-up), and a controlled addressing scheme for reading out data from the scene memory for display in accordance with heading information supplied by the aircraft's on-board navigation equipment. Because the scene memory address signals (which are navigation-dependent) can be expected to be associated with spatial locations in the (terrain map) image that do not necessarily coincide with spatial locations in the map from which the data stored in memory was originally derived, it is necessary to interpolate the data which is used to drive the pixels of the display.
Previously, such interpolation has been accomplished by techniques which include choosing a stored data value associated with that image spatial location closest to the spatial location for the pixel address, or by `truncating` the pixel address to that memory address which is associated with a prescribed one of a set of geometrically dispersed spatial locations adjacent to the spatial location of the pixel of interest. Because such approximations do not take into account variations across the terrain map, such as abrupt color changes, and are limited by the resolution of the database, they may introduce anomalies (e.g. flickering lines) in the displayed image, especially during image rotation, thereby degrading image quality.